Such a device includes at least one scale that extends as a first component of a length measuring device along the associated displacement device, that is, e.g. the first direction, and an associated scanning head for scanning the scale as a second component of the length measuring device, relative to which the scale is movable along the first direction such that a change in the position of the scale with respect to the scanning head along the first direction is measurable, either the scale or the scanning head being moved together with the object to be measured along the first direction and further a device being provided for determining the distance of the component of the length measuring device, which is movable together with the object to be measured, from that object.
German Patent No. 42 19 311 describes a device in the form of a displacement detector that includes a chassis; a contactless sensor (distance sensor) in the form of a focus sensor for measuring the distance between the sensor and a surface (mirror surface) of an object to be measured; a linear scale that is firmly connected to the focus sensor by a connecting part; a device for reading out the graduation marks of the linear scale; an actuator for moving the focus sensor and the linear scale, whereby the linear scale and the focus sensor are moved together relative to the chassis; and a control circuit for operating the actuator.
The linear scale and the associated device for reading out the graduation marks of the linear scale (scanning head) are used to ascertain a movement of the object to be measured, the linear scale being movable together with that object and the associated device for reading out the graduation marks of the linear scale being connected in a stationary manner to the chassis of the displacement detector. Using the contactless sensor provided on the linear scale in the form of a focus sensor as a device for determining the distance between the linear scale and the surface of the object to be measured it is possible to determine the distance between the linear scale and the object to be measured. Consequently, no particularly high demands have to be placed on the tracking of the linear scale when the object to be measured is moved, because the focus sensor associated with the linear scale is able to ascertain the distance between the linear scale and the object to be measured.
A movement of the object to be measured or of the considered surface of the object is ascertained on the basis of the relative movement of the linear scale with respect to the associated device for reading out the graduation marks of the linear scale, it being possible for tracking errors of the linear scale to be corrected (compensated) on the basis of the measuring results of the focus sensor concerning the distance between the linear scale and the object to be measured or its considered surface.
Such a displacement detector has the disadvantage that due to fluctuations of the parameters of the ambient air (air parameter changes), in particular the refractive index, the measuring results of the contactless sensor are subject to uncertainties and are thus not sufficiently exact for high-precision measurements.